1. Field of the Invention
This invention relates to a brushless DC motor. More particularly, it relates to a brushless DC motor which does not have a position sensor for detecting a rotational position of a permanent magnet rotor.
2. Description of the Prior Art
Recently, brushless DC motors have been widely used in industrial audio and video equipment requiring higher reliability for the reasons that service life can be improved as well as noise generation can be reduced resulting from such an advantage that they do not need to have a mechanical contact as would be used in conventional DC motors having brushes.
In order to perform the commutating operation of a conducting phase of the stator windings of a motor, most of conventional brushless DC motors use a rotor position sensor (such as, for example, a Hall effect sensor) instead of using brushes. However, the rotor position sensor itself is not so cheap and requires sophisticated positional adjustment for setting and an increased amount of wiring, so that the cost of the brushless DC motor is disadvantageously high as compared with DC motors having brushes.
In addition, some structural limitations will be frequently imposed thereupon for the reason that a rotor position sensor has to be set inside the motor itself. A recent trend is that accompanied with the minaturization of industrial audio and video equipment, motors to be used therein are made small in size and thickness, which means that the sectional space where a rotor position sensor such as a Hall effect sensor or the like is to be provided becomes extremely small. Under such a circumstance, several types of brushless DC motor having no position sensor such as, for example, a Hall effect sensor have been proposed previously.
Out of which, a brushless DC motor disclosed, for example, in Japanese Laid-Open Patent Application Laid-Open No. 55-160980 is based on the so-called half wave driving method in which an electric current is supplied unidirectionally to the stator windings of the rotor. With this method, counter electromotive forces induced in two stationary stator windings being out of three-phase stator windings are detected, and the signals thus detected are operationally processed to determine the next conducting phase so as to thereby supply an electric current unidirectionally to the stator windings in a successive manner.
Also, disclosed, for example, in Japanese Patent Application Laid-Open No. 62-260586 is a brushless DC motor which is based on the so-called full wave driving method in which an electric current is supplied bidirectionally to the stator windings of the rotor. With this method, the zero-crossing points of the counter electromotive forces induced in the stator windings of the rotor accompanied with the rotation of the motor are detected so as to thereby delay its output signal by a constant period of time by a monostable multi-vibrator; thus, the timing of conducting an electric current is determined.
The former brushless DC motor of the prior art described above (Laid-Open Patent Application No. 55-160980) is based on the so-called half wave driving method in which an electric current is supplied unidirectionally to the stator windings of the rotor, so that its driving circuit can be made simple in structure on the one hand, but on the other hand, the utility and efficiency of the stator windings are low as compared with a brushless DC motor based on the full wave driving method in which an electric current is supplied bidirectionally to the stator windings of the rotor so that a torque to be developed becomes small.
On the other hand, the latter brushless DC motor of the prior art described above (Laid-Open Patent Application No. 62-260586) is based on the method in which the conducting phase is determined by delaying a pulse signal generated at the zero-crossing point of a counter electromotive force induced in each of the stator windings by a constant period of time through a monostable multi-vibrator, so that the delay time is constant independently of the rotational speed of the motor, which, means that it is not stable for an application where the rotational speed has to be changed, thus lacking in its flexibility of application.
In both brushless DC motors according to the prior art described above, an electric current flowing to the stator windings of the rotor is a rectangular waveform signal with a conducting width of about 120.degree. in terms of electrical angle. As a result, in order to reduce an induced spike voltage accompanied with the phase-commutation, a filter circuit including a comparatively large capacitor is particularly required to be provided at a conducting terminal of the, stator windings. Also, an electric current flowing to the stator windings is subjected to an ON-OFF operation in an abrupt manner, so that such a problem has further arisen in that vibration and noise can be easily generated when starting the rotation and this trend is accelerated as the relational speed of the motor is increased.